Ball grid array (BGA) packages for integrated circuits are widely used in the semiconductor packaging industry. Since BGAs use area array contacts, instead of conventional perimeter leads, the physical size of high pin count packages can be significantly reduced. The relatively large pitch size in conventional BGAs requires large package bodies to accommodate high input/output terminal (I/O) applications. To further reduce the size of the packages and accommodate high I/O applications, near chip size BGAs (BGAs which are nearly the size of the integrated circuit that is packaged) have recently been developed.
DiStefano, "Reliable BGAs emerge in micro form", Electronic Engineering Times Pg. 104, 111 (September 1994) discusses a near chip size BGA developed by Tessera.RTM. Inc. (Tessera.RTM. .mu.BGA.TM.). FIG. 1 is a cross-sectional view of a similar BGA 11. In FIG. 1, an integrated circuit 10 has a first surface 22 with a plurality of bonding pads 12 thereon. Bond pads 12 are connected to a plurality of metallizations or traces 16 on a substrate 14 by electrical connectors 18 such as bond wires. Bumps or balls 20 are formed on each trace 16 to enable electrical connections between BGA 11 and other electrical components. A compliant elastomeric layer 15 is located between integrated circuit 10 and substrate 14. Compliant elastomeric layer 15 is attached directly to the first surface 22 of integrated circuit 10 with silicone encapsulant material 19. Integrated circuit 10 is bonded using a layer of adhesive 24 to a thermal spreader 26. Connectors 18 are encapsulated to complete the fabrication of BGA 11.
Compliant elastomeric layer 15 compensates for or takes up the difference in the thermal coefficient of expansion (TCE) between integrated circuit 10 and substrate 14. Compliant elastomeric layer 15 is required because substrate 14, formed of flexible polyimide film, has a relatively high thermal expansion rate compared to the thermal expansion rate of integrated circuit 10. By accommodating for TCE mismatch, compliant elastomeric layer 15 provides protection against stress related problems such as warping and solder ball cracking. However, since compliant elastomeric layer 15 is an elastomer and elastomers are poor thermal conductors, heat transfer from integrated circuit 10 through first surface 22 is inhibited. Yet it is desirable to enhance heat transfer from integrated circuit 10. Further, elastomers have a tendency to absorb moisture from the ambient environment leading to delamination of compliant elastomeric layer 15 and ultimately to the failure of BGA 11.
It is desirable to form an integrated circuit package that is approximately the size of the integrated circuit. Further, as the art moves towards integrated circuits which consume more power, it becomes increasingly important that heat from the integrated circuit package is readily dissipated to the ambient environment. Further, the integrated circuit package should be resistant to environmental degradation.